Spoiler warning: This article contains information on the events of Game of Thrones Season 7, and may potentially spoil the plots of The Winds of Winter and A Dream of Spring. Read at your own peril.

In the final episode of Game of Thrones Season 7, one of Westeros' biggest rats finally ran out of corners to hide in. After attempting to turn Sansa and Arya against one another, Petyr Baelish found himself outwitted and at the mercy of the Stark sisters, who turned out to be terrifically merciless. Arya slit Baelish's throat, executing him for his crimes against her family, which included, among other things: manufacturing the bad blood between the Starks and the Lannisters, betraying Ned Stark, and murdering Lysa and Jon Arryn.

Baelish's death served as a triumphant moment, breaking a tension that had been drawn throughout the season, as viewers wondered whether he would succeed in convincing the Stark sisters to kill one another for their perceived betrayals. But, like many Game of Thrones characters, Littlefinger is still very much alive in the ASOIAF books, and now fans are left to wonder if, when, and how he will meet his end in The Winds of Winter or A Dream of Spring.

A new Game of Thrones fan theory posted to the A Song of Ice and Fire subreddit by redditor rumandraspberries predicts that, if Littlefinger dies in Martin's novels, it will be at the hands of the vengeful Lady Stoneheart:

I think Littlefinger will have a trial, one that's just as heavy handed and one-sided as the one we saw on the show. But it won't be Sansa staring down Littlefinger, it will be Lady Stoneheart. She will be the one to pass the sentence as vengeance for what he did to her family. And his pleads to her will fall on deaf ears, and he'll die terrified and begging for his life in front of a woman he loves, just like in the show.

For those not in the know, Lady Stoneheart is the revenant of Catelyn Stark, who came back to life to seek revenge on those who betrayed her family. Arya acted as a stand-in for Lady Stoneheart in the first episode of Season 7, when she slit the throat of Walder Frey, the man who orchestrated the Red Wedding at which Catelyn, Robb, and their allies were murdered. Lady Stoneheart has already begun to exterminate Frey's family in the books, leading many to believe that she will eventually kill Walder as well.

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If Arya is carrying out Lady Stoneheart's vengeance in Game of Thrones, that definitely means the resurrected Catelyn Stark will be the one to kill Petyr Baelish, right? Wrong. It's likely that neither Arya nor Lady Stoneheart will be the one to kill Littlefinger in the end, because that honor belongs to Sansa herself, at least according to one prophecy in the books.

In A Storm of Swords, Arya and the brotherhood without banners meet with a mysterious seer known as the Ghost of High Heart, in an attempt to locate Beric Dondarrion — A.K.A. That Cool Guy with the Fire Sword, A.K.A. The Man Who Resurrected Lady Stoneheart. The Ghost of High Heart recounts her dreams to them, one of which tells of the Red and Purple Weddings:

I dreamt a wolf howling in the rain, but no one heard his grief. I dreamt such a clangor I thought my head might burst, drums and horns and pipes and screams, but the saddest sound was the little bells. I dreamt of a maid at a feast with purple serpents in her hair, venom dripping from their fangs. And later I dreamt that maid again, slaying a savage giant in a castle built of snow.

The "maid at a feast" is clearly Sansa Stark at Joffrey and Margaery's wedding, where Joffrey was murdered with poisoned wine. The last clause is a little less obvious, but it makes sense if you know that House Baelish's sigil is the Titan of Braavos.